Question

Table A 
(id, name)
1  | alice
2  | bob

Table B
(a_id, last_name)
1    | in wonderland

I want to know the SQL query (using joins) to get the result of “items in A that are not connected to B”

a.2|a.bob

Answer 1

A left outer join will join two tables and return nulls for the second table in the JOIN if there is not a row that matches the join condition. After that you just specify a filter in your WHERE clause indicating that you want only records where the joined table records are null.

SELECT A.id
FROM A
LEFT JOIN B
    ON B.a_id = A.id
WHERE B.a_id IS NULL